Generally, splicing between superconductors is performed in the following cases.
First, short superconductors are spliced to each other for use as a long superconductor for coiling. Second, when connecting superconductor coils, it is necessary to connect superconductor magnet coils to each other. Third, in parallel connection of superconductor permanent current switches for use in permanent current mode (PCM) operation, there is a need to splice a superconductor magnet coil and a superconductor permanent current switch.
Particularly, for superconductor-based devices inevitably designed to operate in a PCM, it is necessary to connect superconductors to function as a single superconductor having perfect continuity and uniformity (physical, chemical, and mechanical). Thus, the superconductors must be operated without any loss of superconductivity after completion of all winding operations.
For example, such splicing between superconductors is performed for superconductor magnets and superconductor-based devices, such as NMR (Nuclear Magnetic Resonance), MRI (Magnetic Resonance Imaging), SMES (Superconducting Magnet Energy Storage), MAGLEV (MAGnetic LEVitation) systems, and the like.
However, since a spliced zone between superconductors generally has inferior characteristics to non-sliced zones in various regards, a critical current (Ic) significantly depends on the spliced zone quality between the superconductors during operation in a PCM.
Thus, improvement of Ic characteristics of the spliced zone between the superconductors is essential in manufacturing of a PCM type superconductor device. However, unlike low temperature superconductors (LTSs), HTSs are formed of ceramic materials, thereby making it very difficult to maintain superconductivity with perfect continuity and uniformity after splicing.
FIG. 1 is a view of a typical 2G ReBCO HTS coated conductor (CC). Referring to FIG. 1, a typical 2G ReBCO HTS 100 is CC comprised of a high temperature superconductor material, such as ReBCO (ReBa2Cu3O7-x, where Re is a rare-earth materials, and x ranges 0≦x≦0.6), and has a laminated tape structure.
A shown in FIG. 1, the 2G ReBCO HTS CC 100 generally includes a substrate 110, a buffer layer 120, a high temperature ReBCO superconducting layer 130, and a stabilizing layer 140.
FIG. 2 schematically shows splicing methods of 2G ReBCO HTS CCs in the related art. There are two alignments, which involve 1) the direct overlap of two superconducting layers of two 2G ReBCO HTS CCs by twisting one CC by 180°, and 2) a patch using a third superconducting layers of a third piece of 2G HTS CC on top of the two superconducting layers of two 2G ReBCO HTS CCs to connect between two 2G HTS CCs aligned in parallel. The advantage of utilizing a third 2G HTS CC piece is the lack of 180° twisting of the two CCs. These two splicing configurations provide flexibility in all ranges of applications, such as stacking and arranging of 2G HTS CC magnet double pancake units.
FIG. 2(a) shows lap joint splicing in which 2G ReBCO HTS 100 are directly spliced to each other. On the other hand, FIG. 2(b) shows overlap joint splicing with butt type arrangement in which 2G ReBCO HTSs 100 are spliced via a third 2G ReBCO HTS 200. Referring to (a) and (b) of FIG. 2, generally, a solder 210 or other normal conductive layer is inserted between surfaces A of the superconductors to splice the 2G ReBCO HTSs.
However, in the superconductors spliced to each other in this manner, electric current inevitably passes through normal conductive (no-superconductive) materials such as the solder 210 and a stabilizing layer 140, which resulted in high resistance, thereby making it difficult to maintain superconductivity of 2G ReBCO HTSs. In the soldering method, a spliced zone can have a very high resistance, ranging of about 20˜2800 nΩ according to the types of superconductor and splicing arrangement.